Overview of Nonpoint Source Reduction Estimate Targets

Artwork used courtesy of USDA.

Nonpoint source loading/reduction estimates are an
integral part of many conservation and nonpoint source water quality improvement projects. Some
types of nonpoint source loadings/reductions are sediment, nutrients,
pesticides, heavy metals and other hazardous chemicals, and microorganisms
(usually associated with fecal contamination of water).
Three common nonpoint source loading/reduction estimate terms used in Virginia are:
(1) Edge of Field; (2) Edge of Stream; and (3) Delivered to Fall Line or directly to tidal
waters as done by the Chesapeake Bay Program.

Important factors impacting edge of field nonpoint source loading/reduction estimates
are soil attributes, type and extent of vegetation, climate, land use, and topography or relief.

Important features impacting edge of stream nonpoint source loading/reduction
estimates are the factors mentioned above for edge of field nonpoint source loading/reduction estimates
as well as distance to stream (hydrologic connectivity), attributes of the land between the field
and stream (slope, soil type, land use, etc.) and other applicable transport factors (transport
from edge of field to edge of stream).

Items impacting delivery to the tidal waters (tidal rivers and the Chesapeake
Bay) nonpoint source loading/reduction estimates are all of the features mentioned above for edge of
field and edge of stream estimates as well as stream attributes, distance to fall line, and
in-stream processes and other applicable transport factors (transport to fall line and tidal
waters).

The matrix of factors affecting nonpoint source loading/reduction estimates is
complex and, usually, modeling is employed to calculate the
loading/reduction estimates. It is also important to note that a water quality improvement
practice (Best Management Practice) implemented at a given site will produce different nonpoint
source loading/reduction
estimates depending on the site specific factors that affect the edge of field estimates and
what target or term is used to describe the estimated overall nonpoint source loading/reduction.
In other words, the nonpoint source loading/reduction estimate value at the edge
of the field will be a different amount than that at the edge of the stream, and both
of these estimates will be different amounts than the reduction estimate at the fall line
for the same implemented practice.